Could be most anything. I was in the apparel business for 24 years before it all went to China, etc. We could have a machine running perfectly on one fabric and change to another and skipping would start. Here are a few things to check:

1. Put in a new needle. You may have a flaw on the old one. Check again that it is threaded correctly. Change to another spool of thread (another color or size).

2. Check that the needle and hook are set and timed right. Something may have moved.

3. Use a presser foot that has as small a hole as possible for the needle to go thru. This can be a major cause for skipping. The fabric may be lifting up into the gap in the presser foot as the needle rises and causing the hook to miss the loop and cause a skip. We called this situation "flagging". We were sewing delicate fabrics. I know that you are usually sewing heavy weight interior fabrics, but a change from leather to a thin headliner fabric could easily result in skipping. Use as small a needle as possible for the fabric you are trying to sew.

4. Find an old guy that once was a mechanic in some sewn product manufacturing plant. He will have it fixed in 10 minutes.